Exploring Light Gas Transport in Carbon Molecular Sieve Membranes Using Diffusion NMR

Carbon molecular sieve (CMS) membranes, microporous membranes fabricated from pyrolysis of polymeric precursors, are promising materials for energy-efficient separations of light gases. The understanding and prediction of transport properties of CMS membranes from only macroscopic observations is a very challenging task due to the complex amorphous pore structure of CMS membranes. Here we report results of an application of a pulsed field gradient (PFG) NMR method for microscopic diffusion studies in CMS membranes. In our multinuclear PFG NMR studies we combine the advantages of using large magnetic field gradients and high magnetic field. This enables obtaining reliable diffusion data even under the conditions of small sorbate diffusivities and short NMR relaxation times. As a result, PFG NMR measurements could be performed for diffusion inside CMS membranes on microscopic length scales of displacements under well-defined, uniform sorption loading and temperature conditions.  The results of these measurements will be presented and compared with the previously reported macroscopic diffusion data.